In the epidermis sphingolipids are concentrated in the upper, differentiated cell layers. Although substantial evidence supports a critical role for sphingolipids in the providion of the permeability barrier, little is known about the regulation of spingolipid biosynthesis in the epidermis. Cultured human keratinocytes should provide a useful model to study the link between spingolipid biosynthesis and epidermal differentiation because these cultures, when grown at an air-medium interface, display advanced morphologic differentiation, as well as increased spingolipid content and biosynthesis in comparison to parallel immersed cultures. In this proposal we will test the following: 1. Sphingolipid composition and synthesis by keratinocytes varies in relation to their degree of differentiation. 2. Products of sphingolipid metabolism may themselves regulate epidermal differentiation.